It was the new Andy Murray. Gone was the surly, wild-haired young man we saw at Wimbledon. In his place at the Australian Open last week was a smart, smiling and confident star-in-the-making, charming the media and winning a legion of admirers on and off the court.

On Sunday morning the new Murray was fighting to change the record books, taking on Roger Federer and attempting to become the first British man for 74 years to win a Grand Slam singles title.

Tennis insiders said Murray's transformation could be put down to several factors: his new manager Simon Fuller, the fitness regime introduced by Team Murray – including Bikram yoga – and the ending of the relationship with his long-term girlfriend, Kim Sears.

Many in the game were surprised last year when Murray signed up with Fuller's 19 Entertainment rather than a global sports agency such as IMG, which boasts clients including Federer and Tiger Woods.

The pop impresario, who created the Spice Girls and manages David Beckham, Claudia Schiffer and Katy Perry, set about the rebranding of Murray with gusto when he won the contract.

He has given the player a new look, including a decent haircut and wardrobe of adidas clothes. He has softened the Scottish phenomenon's attitude to the media.

The £15 million five-year deal with adidas – replacing a contract with Fred Perry – is part of a tough new commercial approach that will bring in promotional deals aimed at propelling Murray to global stardom.

His advisers were said to be aghast when they discovered that the shirts he wore were not available in the shops.

A Grand Slam win will turn Murray into one of Britain's highest earning sportsmen, on target to make £50 million a year.

Team Murray has also seen some significant changes. His mother Judy Murray, who encouraged him to pick up a racket at two and helped him to go to an academy in Spain at 15, is still there, cheering him on at every shot.

But Miss Sears, his girlfriend of four years, is not. The pair split up before Christmas, and some fans say that since then Murray has become more relaxed and has rediscovered the joys of being young and single – with a positive effect on his tennis.

The day after he announced they had parted Murray spent the evening partying with friends in the VIP room of Tiger Tiger in London's Haymarket, declaring: "I'm single and out with the boys."

Since then he has given another glimpse of his racier side by dipping into the £2.25 million he won in prize money last year to buy a £100,000 second hand red Ferrari F430.

It is rumoured that he has started redecorating the £5.5 million home he once shared with Miss Sears in Oxshott, Surrey. His friendship with Miss Scotland, the model Katherine Brown, has set tongues wagging.

Team Murray has been pared down, the unwieldy entourage cut to just three: Miles Maclagan, the coach; Jez Green, the fitness trainer behind the Murray "biceps" who was hired after he split with Brad Gilbert, his former coach, in November 2008; and Andy Ireland, the physiotherapist.

Another factor in the making of the new, chilled-out Mr Murray is his friendship with Laura Robson, the 16-year-old British tennis star who is competing in the doubles.

Crucially, however, Murray's training regime has changed, creating a more muscular, more physical player with stamina and aggression to burn.

Thanks to his fitness consultants he has also become a devotee of Bikram yoga, said to be the world's fastest growing yoga, which is done in a room heated to 42C (107F) and is designed to increase flexibility and strength, and prevent and heal injuries.

Fuller insists that most of Murray's improvement is down to his own extraordinary determination to become the world's best tennis player.

"Andy has matured a lot in the past 12 months," he said. "His confidence and self-belief is impressive for his age. He's an intelligent player and his all-round game is improving at a spectacular rate.

In between games in Melbourne Murray has kept a diary on Twitter, revealing that he is a big fan of Jedward, the X Factor singers, and that he received well done messages from Chris Hoy, the Olympic cyclist, and the boxer Ricky Hatton.

But Murray's new relaxed demeanour hides a steely determination to win his first Grand Slam. Never a party animal, he has been teetotal since he fell ill after drinking alcohol six years ago.

As Fuller says: "He will make any sacrifice to be the world's number one."

The BBC declined to predict the audience for Sunday's final but a spokesman said there was “huge interest” in the match and hoped that the figure would beat the 11.1 million who watched last year’s Wimbledon final, in which Federer beat Andy Roddick in five sets.